Among the various ceramic materials having high dielectric constants, barium titanate (BaTiO.sub.3) and a solid solution of, typically, barium titanate (BaTiO.sub.3), calcium titanate (CaTiO.sub.3) and lead titanate (PbTiO.sub.3) in particular have conventionally been used for practical purposes, as well known in the art. These known dielectric ceramic materials are however not fully acceptable for their dielectric constants which are of the order of as low as 8000 or even less. Increasing the dielectric constants of any of such known ceramic materials at an ambient temperature will contribute to improvement in some electrical characteristics of, for example, a capacitor using the material. However, such an attempt would results in objectionable broadening of the range over which the dielectric constant of the material is variable with temperature. If, then, one attempts to restrict the range of variation of the dielectric constant with temperature, there would result reduction in the maximum value of the dielectric constant.
On the other hand, it is also known that ceramic materials for ordinary use are required to have low dielectric loses and high insulating resistances. As to the latter, it is desirable that ceramic materials for use in ceramic capacitors have high insulating resistances not only at room temperature but also at the highest temperatures at which the capacitors may be required to operate. This is also required by the standard (MIL-C-555681B) established by the Military Specification of the U.S. Department of Defense.
As noted at the outset of the description, a ceramic composition with a high dielectric constant is useful particular as a dielectric material of a multilayer ceramic capacitor. Such a multilayer ceramic capacitor may be fabricated on a semiconductor substrate to form a chip capacitor. In a chip capacitor of this nature, there may be a difference in thermal expansion coefficient present between the semiconductor substrate and the bulk ceramic forming part of the capacitor structure. Such a difference in thermal expansion coefficient may create a mechanical strain in the capacitor structure so that cracks and even serious damages resulting from the cracks may be produced in the capacitor structure due to the mechanical strains. On the other hand, a ceramic composition with a high dielectric constant may be used to form part of a dip (dual in-line package) capacitor which is typically packaged in an epoxy resin. In the case of a capacitor of this type, a similar problem may arise due to the cracks created in the capacitor structure due to the stress imparted to the structure from the packaging material.
In any event, the lower the mechanical strengths of the ceramic materials forming part of capacitor structures, the more frequently will cracks and damages resulting therefrom be created in the capacitor structures and accordingly the lower the reliability of performance of the capacitor will become. Increasing the mechanical strength of a ceramic material is, thus, one of the major practical requirements of the material especially where the material is intended for use in a chip of dip-type capacitor.
It is accordingly an important object of the present invention to provide an improved ceramic composition having a high dielectric constant and a low dielectric loss.
It is another important object of the present invention to provide an improved ceramic composition having a high insulation resistance at room and higher temperatures in addition to the high dielectric constant and low dielectric loss.
It is still another important object of the present invention to provide an improved ceramic composition which not only has a high dielectric constant and a low dielectric loss but exhibits a high insulation resistance at room and higher temperatures and excellent mechanical properties.
Yet, it is still another important object of the present invention to provide an improved ceramic capacitor which features enhanced performance reliability for its increased resistance to cracks and damages.